This invention relates to frequency standards, rubidium frequency standards, and more specifically to temperature compensation of a rubidium frequency standard.
As hopping rates increase in spread spectrum radios such as JTIDS, LINK 16, HAVE QUICK, SINCGARS, and SATURN, time of day clocks are required to become more accurate to insure quick synchronization and longer mission times without synchronization. These radios require very accurate frequency standards that are pressing the limits of oven controlled crystal oscillators (OCXO) and temperature compensated crystal oscillators (TCXO).
Rubidium frequency standards have been known in the art for many years. Rubidium frequency standards provide greater accuracy than OCXO and TCXO frequency standards. Rubidium frequency standards operate by locking a crystal oscillator to a hyperfine transition at 6.834,682,612 GHz in rubidium. The amount of light from a rubidium discharge lamp that reaches a photo detector through a rubidium gas resonance cell is reduced when rubidium vapor in the resonance cell is excited by a microwave signal near the transition frequency. The crystal oscillator is locked to the rubidium transition by detecting the light output drop when sweeping an RF frequency synthesizer containing the crystal through the transition frequency.
Rubidium frequency standards are useful in spread spectrum radios and such systems as GPS to provide high frequency accuracy. However, rubidium frequency standards are large and sensitive to changes in temperature that cause changes in frequency. Rubidium frequency standards also consume large amounts of power.
In a rubidium frequency standard a gas cell, light source, and photo detector are all temperature sensitive and each requires a specific temperature for optimal operation. The optimal temperature for the three devices is not the same so the temperature used is a compromise of the desired temperatures, making the devices more sensitive to external temperature change. The frequency of the rubidium transition is sensitive to the pressure in the gas cell, which changes with temperature. Rubidium""s melting point is 34xc2x0 C. so for rubidium to be in the gaseous state, the temperature of the gas cell must be elevated with an oven. Since there are two different isotopes of rubidium there are two different resonances. A filter cell is used to filter out one resonance. A second cell is used to absorb the light of a rubidium lamp or laser diode. This abortion point is very fine providing a high Q frequency reference. For proper operation the filter cell and the absorption cell must be at a specific temperature and the lamp has its specific temperature to provide a zero light shift/zero temperature coefficient (TC) condition (ZLS/ZTC). Deviation from these ideal temperatures means the rubidium frequency standard becomes more dependent on temperature changes. Different types of light sources have been tried in an effort to minimize the effect of the light shifting.
The TC of the rubidium frequency standard is closely related to how well the ZLS/ZTC temperature can be maintained. As the frequency standard is miniaturized, the cells and the lamp are brought closer meaning their optimal temperatures must be compromised. Also the room for thermal insulation is minimized. In addition the electronics components are brought closer together, concentrating their heat, causing thermal gradients. All this makes it much more difficult to maintain the optimal temperature conditions required for frequency stability.
Previous attempts to stabilize rubidium frequency standards have included ovenization of the components and for even higher stability units double ovenization or cooling by thermal electric cooling. An OCXO is typically used to provide the output frequency. All these attempts occupy large volumes and consume high power in the rubidium frequency standard.
For many applications in GPS and radio communications systems miniaturization and low power consumption are required. The high stability of a rubidium frequency standard is also required. What is needed is a miniaturized rubidium frequency standard with temperature compensation to offer high stability without consuming large amounts of power.
A rubidium frequency standard with a temperature compensated output signal is disclosed. The rubidium frequency standard has a voltage controlled crystal oscillator (VCXO) that provides the temperature compensated output signal. A frequency synthesizer receives the output signal from the voltage controlled crystal oscillator as a reference and provides a RF signal. A physics package receives the RF signal and provides a light output signal with a null indicating when the RF signal is such that a transition frequency of rubidium is obtained within the physics package. A temperature sensor senses temperature and provides a temperature signal. A microcontroller receives the light output signal, generates a control word for the frequency synthesizer, receives the temperature signal, and provides an error signal to the VCXO to lock the VCXO to the physics package. The microcontroller uses the temperature signal to look up a frequency error in a memory to offset the synthesizer and adjust the VCXO to compensate for temperature.
The physics package further comprises a multiplier to multiplying the RF signal to a microwave frequency, a light source for providing light, a gas cell excited by the microwave frequency that passes the light from the light source and reduces the light from the light source when the gas cell is excited by the microwave frequency at the transition frequency. A photo detector detects the light passed through the gas cell and provides a light output signal that is at the null when the gas cell is excited at the transition frequency.
The microcontroller calculates a offset control word for the frequency synthesizer by use of a temperature versus control word lookup table, sweeps the synthesizer around the offset control word while sampling the photo detector output, detects the null in the photo detector output, and adjusts the VCXO frequency so that the null frequency matches the synthesizer setting that is recorded in the memory. A compensation table is built and stored in the memory that contains the offset control word for the temperature.
During temperature compensation of the rubidium frequency standard a high stability reference oscillator is substituted for the VCXO, the rubidium frequency standard is run over a desired temperature range, and the microcontroller adjusts the synthesizer with a control word to find a null point in the photo detector output and then records the offset control word along with the temperature at which the offset control word was found in the compensation table.
It is an object of the present invention to compensate for frequency variations over temperature in a rubidium frequency standard.
It is an object of the present invention to correct for frequency variations over temperature in a rubidium frequency standard by allowing the rubidium frequency to varying while maintaining a constant output frequency.
It is an advantage of the present invention to allow size reduction of a rubidium frequency standard while maintaining frequency accuracy.
It is an advantage of the present invention to eliminate an oven controlled crystal oscillator.
It is an advantage of the present invention to reduce power consumption in a rubidium frequency standard,
It is a feature of the present invention to allow removal of insulation in ovenized components.
It is a feature of the present invention to provide improved frequency accuracy in a compact package with reduced size for radio communications and GPS applications.